Corrugated paperboard is discharged from a corrugator machine as a continuous web which normally travels at substantial velocities. As a web travels, it is operated on by various machines for conversion into articles of manufacture such as corrugated cartons and the like. One such machine operating on the web is a slitting and scoring machine which severs the moving web from the corrugator lengthwise as well as scoring the web lengthwise. The respective webs exiting from the slitting and scoring machine then must be severed into sheets or blanks.
The lengths of the sheets severed from one web may be different from that of sheets severed from the other webs. Accordingly, one or more cutters must be utilized. Typically, rotary cutters or shears are used to sever the traveling paperboard webs more or less perpendicular to their directions of travel for production of sheets or blanks. In sheet production, it is desirable that the web be severed as near perpendicular to its direction of travel as possible. Also, in sheet production, it is desirable to control the rotary cutter such that sheet length is determined with a high degree of precision and with essentially no variance between different sheets.
There are basically two types of rotary shears and cutters, straight knife cutters and helical knife cutters.
The principal disadvantages of the existing straight knife rotary shear and cutter machinery are as follows:
(1) Straight knives are used for severing the web and consequently the entire web must be severed in a single simultaneous cut across its entire width;
(2) Straight knives require extremely massive carrier shafts or knife bars to withstand the impulse load of a single simultaneous cut across the entire web width;
(3) Existing rotary shears/cutters require complex mechanical drives and transmissions for cyclically driving the massive knife shafts which must be capable of withstanding the considerable stresses experienced in so driving such massive knife shafts;
(4) The mechanical drives and transmissions must include mechanical means for adjusting the cyclic rotation period of the knife shafts to allow production of sheets or blanks of different lengths;
(5) The range of sheet lengths which can be cut without decreasing web velocity is limited;
(6) Existing rotary shears/cutters are inertially limited to operate at low web velocities because of the massive components (transmissions and knife shafts).
For additional discussion of other problems and disadvantages of conventional straight knife mechanically controlled rotary shears/cutters, see U.S. Pat. Nos. 3,748,865 (R. C. Johnson) and 3,003,380 (H. W. Moser), each describing very complex mechanical means for delivering power to the rotating knife shafts and controlling the cyclic rotation period of such shafts.
Other serious disadvantages of existing straight knife rotary shear/cutter machines relate to their short lifetime, i.e., the mechanical vulnerability of component parts. Such mechanical vulnerability is primarily due to the massive nature of the components, cyclic loading of the gear train, and the impact loading experienced by the components upon each cut or shear. On the average, a conventional straight knife rotary shear/cutter must have one or another of its major components repaired or replaced each year. Specific components which are prone to mechanical failure are the transmission drives with sliding cranks and the like. Counterbalanced cut-off drives, while eliminating many of the problems inherent in sliding crank cut-off drives, introduce additional mass to the drive assembly which must be rotationally driven, thus creating inertial limitations on the operation and control of such machines. A good discussion of the problems encountered in producing and controlling sheet length cut by conventional straight knife rotary cutters is presented in U.S. Pat. No. 4,034,635 (A. B. Woolston).
Finally, conventional straight knife rotary shears/cutters for severing two webs exiting from a slitter/scorer machine typically have an extremely massive frame occupying considerable floor space.
The principal disadvantage of rotary shears/cutters carrying helical blades are as follows:
(1) Existing helical knife rotary shears/cutters do not sever the web perpendicular to its direction of travel;
(2) It is difficult to maintain proper alignment of the helical blades for progressively shearing material along their entire lengths;
(3) When driven by mechanical drives and transmissions, such must include mechanical means for adjusting the cyclic rotation period of the knife shafts to allow production of sheets or blanks of different lengths;
(4) When driven by independent drives, there is difficulty in obtaining uniform sheet lengths; and
(5) Existing systems are inertially limited to operate at relatively low velocities because of their massive components (transmissions and knife shafts).
For a discussion of rotary shears/cutters carrying helical blades, see U.S. Pat. Nos. 3,570,348 (K. W. Hallden), 1,318,892 (M. Maier), and 2,942,534 (C. A. Boddy).
Consistent production of sheets of the same length with either straight knife or helical knife rotary shears/cutters is desirable but extremely difficult to obtain. Specifically, the web does not move at a constant velocity but rather at a variable velocity in part dependent upon the velocity at which the corrugator unit produces the paperboard web, in part dependent on other components operating on the web and in part dependent on the physical properties of the type of web material being produced.
In mechanical drive systems following the corrugator drive, the inertia of the drive components preclude accurate sheet length control. Moreover, in mechanical systems which incorporate several mechanisms for implementing corrections in sheet length control after error has been detected, the servo response systems typically have long time constants related to the expected linear velocity of the web material being severed successively (see U.S. Pat. No. 4,034,635 [A. B. Woolston]). Because of the inability to obtain precise sheet length control with mechanically linked drive systems for rotary shears, various electronically controlled independent direct drive rotary shears/cutters have been developed.
The essential components of such a direct drive system include a reversible drive capable of accelerating and decelerating rotation of the knife shafts, an electronic means for generating electronic pulses bearing a predetermined relationship to the velocity of the web prior to its being severed, an electronic means for generating electronic pulses bearing a predetermined relationship to the angular position and velocity of the shafts carrying the knives, a cut completion sensor for generating an electronic pulse signal indicating completion of a cut, and typically, a digital logic control system comparing the electronic signals received from said sensors for regulating the reversible drive. For a description of an existing digital logic control system for regulating direct drive rotary shears/cutters for sheet production, see U.S. Pat. No. 4,020,406 (Tokuno, et al., Col. 7, 1. 45-Col. 10, 1.5).
The weaknesses of existing direct drive rotary shears/cutters relate primarily to the components sensing web velocity. In particular, it is extremely difficult to accurately sense web velocity at one point in a web and generate electronic pulses corresponding thereto as a function of time and correlate it to the velocity at which the web is moving to the point it is to be severed. This problem is particularly acute since the knives, whether helical or straight, carried by the rotating shafts must be moving at the velocity of the web at the point they engage the web throughout the period of the cut, i.e., until they rotate out of the path of the web. For example, if the blades are traveling at a velocity greater than that of the web, a sheet may be torn from the web rather than severed from the web. Conversely, if the blades are moving slower than the web through the period of cut, then the web upstream will buckle against the blades during the period of cut. Accordingly, the next sheet cut will have a greater length than the sheet previously cut.
Another serious drawback in existing digitally controlled direct drive rotary shears/cutters relates to spurious electronic pulse generation in the control electronics. Spurious pulses result from switching from one control circuit to another and like phenomenon. Such spurious electronic pulses can cause the digital control system to operate out of a synchronism with the web velocity, thus precluding accurate sheet length control.
Finally, most other machinery units necessary for manufacturing paperboard have capacities to operate at much higher web velocities than existing rotary shears/cutters. In fact, it is often necessary, because of the mechanical vulnerability of existing rotary shears/cutters, to operate machinery both up and downstream from the cutter to less than half their operational capacity. However, for economical operation, the machinery manufacturing corrugated paperboard must operate at or near full capacity. Accordingly, any component of the machinery system operating on the paperboard web which limits the velocity of the web to less than the velocity at which the corrugator produces the web is detrimental.